This invention pertains to apparatus for separating at least one fluid from a fluid mixture containing at least one other component by selective permeation of the at least one fluid through membranes.
The use of membranes for separating at least one fluid from a fluid mixture containing at least one other component has long been suggested. In these membrane separations, permeable fluids in the fluid mixture (feed mixture) pass, under the influence of a driving force such as concentration, partial pressure, total pressure, etc., (depending on the nature of the membrane and separation operation) from a feed side of the membrane to a permeate side of the membrane. The fluid may pass through the membrane by interaction with the material of the membrane or by flow in interstices or pores present in the membrane. Separations effected by membranes can include gas-gas, gas-liquid, and liquid-liquid (including liquid-dissolved solid) separations.
The viability of the use of membranes for fluid separations as compared to other separation procedures such as absorption, adsorption, distillation, and liquifaction often depends on the cost including installation and operating costs, the degree of selectivity of separation which is desired, the total pressure losses caused by the apparatus for conducting the separation procedure which can be tolerated, the useful life of such apparatus, and the size and ease of use of such apparatus. Film membranes may frequently not be attractive as other separation apparatus due to the need for film membranes to be supported to withstand operating conditions and the overall complexity of apparatus containing film membranes. Membranes in the configuration of hollow fibers, or hollow filaments, may overcome some of the deficiencies of film membranes for many separation operations in that the hollow fibers are generally self-supporting, even during operating conditions, and provide a greater amount of membrane surface area per unit volume of separation apparatus than that which is provided by film membranes. Thus, separation apparatus containing hollow fibers may be attractive from the standpoint of convenience in size and reduced complexity of design.
The two approaches which have primarily been proposed for apparatus for fluid separations employing hollow fiber membranes use a plurality of the hollow fibers generally parallelly arranged in an elongated bundle. One approach includes transversely-fed permeators, e.g., radially-fed permeators, in which separation apparatus the fluid at the exterior of the hollow fibers primarily passes through the permeator transverse to the predominant longitudinal orientation of the hollow fibers. The other approach includes axially-fed permeators, e.g., separation apparatus wherein the fluid at the exterior of the hollow fibers primarily passes through the permeator in the same direction as the longitudinal orientation of the hollow fibers; however, there must be some transverse flow of fluid such that the fluid is distributed in the bundle. Transverse flow of the fluid among the hollow fibers in axially-fed permeators is at least partially due to dispersions caused by back pressures to fluid flow in the permeator, that is, the path of least resistance for the fluid flow must be transverse to the orientation of the hollow fibers in order to obtain the desired transverse flow. The efficiency of separation for a hollow fiber-containing permeator is dependent on the effective fluid distribution of the fluid at the exterior of the hollow fibers. In some instances, transversely-fed permeators can exhibit greater efficiencies of separation than do axially-fed permeators. In a transversely-fed permeator, the fluid must transversely pass among the hollow fibers, and thus dispersion of the fluid through the bundle is effected using the primary direction of flow of the fluid. On the other hand in axially-fed permeators, the fluid dispersion among the hollow fibers in the permeator is not so ensured. Despite the deficiencies which may exist in fluid dispersion around the exteriors of the hollow fibers in axially-fed permeators, such permeators offer significant advantages in terms of reduced construction difficulties and complexity over transversely-fed permeators. Another significant advantage of axially-fed permeators is the benefit of cocurrent or countercurrent flow patterns that can be obtained with respect to the passage of fluid in the bores and at the exteriors of the hollow fibers.
While the efficiency of separation of axially-fed permeators might be enhanced by introducing the feed mixture into the bores of the hollow fibers, especially in separation operations in which no sweep fluid is utilized on the permeate side of the membrane, this procedure is often undesirable due to the significant pressure drops which might be encountered in the passage of the fluid mixture through the bores of the hollow fiber. Even with bore feed, some sacrifice in separation efficiency may still occur due to poor distribution of permeate, for instance, localized regions, or pockets, of permeate on the shell side of the hollow fibers. Moreover, in some separation operations it is desirable to provide the feed mixture at the exterior of the hollow fibers since the feed mixture is at a higher total pressure than the total pressure on the permeate side of the membrane. Furthermore, if fouling of the membrane due to components in the feed mixture is a problem, less available membrane surface area may be lost where the fouling is at the exterior of the hollow fibers rather than blocking or partially blocking any of the bores of the hollow fibers. Additionally, the fabrication of the permeator may be facilitated since when the fluid mixture containing the fluid to be separated is contacted with the exterior of the hollow fibers, often only one end of each of the hollow fibers need be fabricated into a seal, or tube sheet, to enable fluid communication from the bores of the hollow fibers to the exterior of the permeator. Accordingly, a means to enhance the efficiencies of separation of axially-fed permeators are sought.
The efficiencies of separations using axially-fed permeators can adversely be affected by the formation of longitudinal flow channels. Often channeling can be observed in the region between the shell and the bundle. These flow channels may be formed by, for instance, the movement of the hollow fibers caused by the forces of fluid flow or by gravity (e.g., settling). The fluid on the shell side of the hollow fibers will preferentially pass through the longitudinal flow channels as opposed to dispersing among the hollow fibers since the total pressure drop incurred by the fluid passing through the channel is generally inversely proportional to the diameter of the channel to the fourth power. Thus, even small increases in the diameter of the channel can result in substantial reduction of pressure drop to the fluid flow. Hence, the transverse dispersion of the fluid will be adversely affected. Accordingly, losses in efficiencies of separation can result.
One method for reducing the effect of channeling is to increase the number of hollow fibers in the permeator such that deleterious channels can not readily form. However, as the number of hollow fibers increases the difficulty of insertion of the hollow fibers into the shell of the permeator also increases. Clearly, the insertion of the hollow fibers into the shell must be conducted without incurring undue damage to the hollow fibers. Moreover, even slight increases in packing factors caused by increasing the number of hollow fibers in the permeator have been found to provide much greater resistances to fluids dispersing in a transverse direction among the hollow fibers. Maxwell, et al., in U.S. Pat. No. 3,339,341, disclose a hollow fiber separation apparatus in which the fluid mixture containing the fluid to be separated is fed to the bores of the hollow fibers. The patentees indicate at column 5, lines 24 et seq., that packing densities over 40 percent are preferred and that at these packing densities do not prevent all movement of fluid into and out of the bundles between hollow fibers, but they do cause the fluids outside of the hollow fibers to flow along and in the direction of the hollow fibers. This objective may thus be contrary to obtaining good radial dispersion of fluids among hollow fibers in a permeator. Maxwell, et al., state that the use of a sweep gas at the exteriors of the hollow fibers is a preferred mode of operation (see column 7, line 5, et seq.). Apparently the radial distribution desired by Maxwell, et al., when employing a sweep fluid is fostered by utilizing a plurality of spaced apart sub-groups, or sub-bundles, such that the sweep fluid could radially pass between the sub-bundles. Consequently, in view of the lesser diameter the sweep fluid can more readily disperse into the mid-portion of a sub-bundle than from the outside into the mid-portion of a bundle not containing these sub-bundles. Unfortunately, the preparation of sub-bundles requires additional processing steps in the fabrication of separation apparatus and also channels may exist between sub-bundles which reduce the efficiency of separation of the separation apparatus.
In view of the difficulties in fabricating axially-fed permeators which exhibit commercially desirable efficiencies of separation, it is not surprising that little research and development efforts have been expended on the fabrication of axially-fed permeators. Rather, significant efforts have been devoted to the fabrication of, e.g., transversely-fed permeators in which the development of high efficiencies of separation can be more assured, especially if the feed mixture is to be fed to the exteriors of the hollow fibers. However, as pointed out above, axially-fed permeators can provide significant advantages over radially-fed permeators if non-complex means are provided to reliably increase the efficiencies of separations exhibited by axially-fed permeators.
In the copending patent application of Bollinger, U.S. patent application Ser. No. 961,343, filed on the same date herewith, there is described axially-fed permeators in which the efficiencies of separation exhibited by the permeators are increased by utilizing barrier bands surrounding the bundle of hollow fibers. The barrier band is adapted to provide a localized zone of increased lateral compaction of the bundle. While the fabrication of axially-fed permeators containing barrier bands may be relatively straightforward, the steps of fashioning the barrier band, providing the barrier band around the bundle, and providing the laterally compacted zone must be accomplished.
By this invention axially-fed permeators containing hollow fiber separation membranes for separating by the selective permeation of at least one fluid from a fluid feed mixture containing at least one other component are provided which permeators can exhibit highly desirable efficiencies of separation due to enhanced dispersion of fluid at the exterior of the hollow fibers. Advantageously, the axially-fed permeators of this invention can be fabricated employing large number of hollow fibers closely adjacent to one and another to make efficient utilization of the volume of the permeators; however, the permeator can be fabricated without undue difficulties or risk of damage to the hollow fibers. The benefits of the permeators of this invention can be achieved with relatively non-complex modifications of existing axially-fed permeator designs. Moreover, many existing axially-fed permeators may be readily modified to provide permeators in accordance with this invention which exhibit enhanced efficiencies of separation.